1Department of Pharmaceutical Sciences, Room No. HGH-390, 372 Fairfield Road, University of Connecticut, Storrs-CT, 06269 USA
2School of Biotechnology, Faculty of Science, Banaras Hindu University, Varanasi 221 005, India
*Corresponding author. E-mail: srivastava@uconn.edu
Pigeonpea (Cajanus cajan) urease was inactivated by incubating it in buffer of low pHs i.e., 4.8 and 4.5. The pattern of inactivation at both pHs was found to be biphasic, in which half of the activity was destroyed more rapidly than the remaining half. This distribution of active site into two categories is suggestive of site-site heterogeneity, or more specifically, the half-site reactivity of the enzyme moiety. Our pH studies on the rate of reaction showed the presence of two ionizable groups of ρKa values 6.2 ± 0.1 and 8.8 ± 0.1, respectively (Srivastava PK & Kayastha AM, J Mol Catal B: Enz, 16 (2001) 81–89). The later group corresponds to the ρKa value of cysteine group. Here we correlate the loss of urease activity by low pH treatment is due to the effect on essential thiol residues.
urease, pigeonpea, pH inactivation, thiol residues, half-site reactivity, biphasic kinetics
